1. Field of the Invention
This invention relates to preheaters and particularly to an air or other gas preheater suitable for a laboratory furnace. The preheater according to the present invention provides improved furnace temperature uniformity and atmosphere control.
2. Brief Description of the Prior Art
Ashing of coal and coke samples in proximate analysis pursuant to procedures prescribed in ASTM D3172-73 (Reapproved 1979), D3174-73 (Reapproved 1979) and proposed revision for D3174-73 (subject to ballot at D.5.21.01 level as of Jan. 7, 1980) requires that air be introduced into a furnace chamber at the rate of 2 to 4 atmosphere changes per minute. Present methods for meeting this requirement are crude. One method is simply to open the furnace door about 1/2 inch, which allows air to diffuse into the furnace chamber. Another method employs a suction device for drawing ambient air directly into the furnace chamber. These methods are not completely effective, as pointed out in the text of the revision procedure. First, they destroy furnace temperature uniformity; at a nominal temperature of 750.degree. C., the actual temperature may vary as much as .+-.100.degree. C. Second, these methods produce so-called "shadow" zones in the furnace chamber. By "shadow" zones is meant areas which are simply devoid of air.